Enzyme therapy. II. Purified human alpha-galactosidase A. Stabilization to heat and protease degradation by complexing with antibody and by chemical modification

Stabilization of invertase by molecular engineering

Extracellular invertase (EC 3.2.1.26) of Saccharomyces cerevisiae was stabilized against thermal denaturation by intermolecular and intramolecular crosslinking of the surface nucleophilic functional groups with diisocyanate homobifunctional reagents (O==C==N(CH(2))(n)N==C==O) of various lengths (n = 4, 6, 8). Crosslinking with 1,4-diisocyanatobutane (n = 4) proved most effective in enhancing thermostability. Stability was improved dramatically by crosslinking 0.5 mg/mL of protein with 30 mumol/mL of the reagent. Molecular engineering by crosslinking reduced the first-order thermal denaturation constant at 60 degrees C from 1.567 min(-1) (for the native enzyme) to 0.437 min(-1) (for the stabilized enzyme). Similarly, the best crosslinking treatment increased the activation energy for denaturation from 391 kJ mol(-1) (for the native protein) to 466 kJ mol(-1) (for the stabilized enzyme). Crosslinking was confirmed by sodium dodecyl sulfate polyacrylamide gel electrophoresis.

Antibodies as specific chaperones

Protein folding is often accompanied by formation of non-native conformations leading to protein aggregation. A number of reports indicate that antibodies can facilitate folding and prevent aggregation of protein antigens. The influence of antibodies on folding is strictly antigen specific. Chaperone-like antibody activity may be due to the stabilization of native antigen conformations or folding transition states, or screening of aggregating hydrophobic surfaces. Taking advantage of chaperone-like activity of antibodies for immunotherapy may prove to be a promising approach to the treatment of Alzheimer's and prion-related diseases. Antibody-assisted folding may enhance renaturation of recombinant proteins from inclusion bodies.

Enhanced stability of beta-galactosidase in parenchymal and nonparenchymal liver cells by conjugation with dextran

In order to enhance the stability of beta-galactosidase, we conjugated the enzyme with dextran T-10 (Mr approx. 10 000). The conjugate contained 9-10 mol dextran/mol protein (beta-galactosidase, Mr 68 000), and the specific activity retained after conjugation was 90 +/- 4% (n = 3) of the initial activity. Uptake and degradation of native and conjugated beta-galactosidase in isolated hepatocytes and nonparenchymal liver cells was studied. There was a marked increase in stability against degradation in both cell types when beta-galactosidase was conjugated with Dextran. The degradation of dextran-conjugated enzyme was reduced by 35% in hepatocytes and by 43% in nonparenchymal cells, after 80 and 40 min, respectively, as compared with the free enzyme. However, there was insignificant difference between the uptake of native and conjugated enzyme into the liver cells. Upon intravenous infusion into rats, native and conjugated enzyme were cleared from plasma with only a slight difference in the clearance rate. The observed stability of dextran-conjugated beta-galactosidase towards cellular degradation was in accordance with the in vitro experiments. The conjugate showed marked thermal stability at 50 degrees C and enhanced resistance towards proteolysis by the broad specific protease subtilopeptidase A. This demonstrates that dextran conjugation may be used as a means of stabilizing lysosomal enzymes for therapeutic purposes.

Basic findings and current developments in sphingolipidoses

Sphingolipidoses are caused by recessively inherited deficiencies of lysosomal hydrolases. The clinical backgrounds of and current biochemical and genetic approaches to the different forms and variants of gangliosidoses, trihexosylceramidosis (Fabry's disease), galactosylceramidosis (Krabbe's disease), sulfatidoses (metachromatic leukodystrophies), glucosylceramidosis (Gaucher's disease), sphingomyelinoses (Niemann-Pick disease) and ceramidosis (Farber's disease) are presented.

Biochemistry and genetics of gangliosidoses

The gangliosidoses comprise an-ever increasing number of biochemically and phenotypically variant diseases. In most of them an autosomal recessive inherited deficiency of a lysosomal hydrolase results in the fatal accumulation of glucolipids (predominantly in the nervous tissue) and of oligosaccharides. The structure, substrate specificity, immunological properties of and genetic studies on the relevant glycosidases, ganglioside GM1 beta-galactosidase and beta-hexosaminidase isoenzymes, are reviewed in this paper. Contrary to general expectation, only a poor correlation is observed between the severity of the disease and residual activity of the defective enzyme when measured with synthetic or natural substrates in the presence of detergents. For the understanding of variant diseases and for their pre- and postnatal diagnosis, the necessity of studying the substrate specificity of normal and mutated enzymes under conditions similar to the in vivo situation, e.g., with natural substrates in the presence of appropriate activator proteins, is stressed. The possibility that detergents may have adverse affects on the substrate specificity of the enzymes is discussed for the beta-hexosaminidases. The significance of activator proteins for the proper interaction of lipid substrates and water-soluble hydrolases is illustrated by the fatal glycolipid storage resulting from an activator protein deficiency in the AB variant of GM2-gangliosidosis. Recent somatic complementation studies have revealed the existence of a presumably post-translational modification factor necessary for the expression of ganglioside GM1 beta-galactosidase activity. This factor is deficient in a group of variants of GM1-glangliosidosis. Among the possible reasons for the variability of enzyme activity levels in heterozygotes and patients, allelic mutations, formation of hybrid enzymes, and the existence of patients as compound heterozygotes are discussed. All these may result in the production of mutant enzymes with an altered specificity for a variety of natural substrates.

Kinetics of uptake and activity in mouse liver of glutaminase coupled to desialated orosomucoid

Desialised orosomucoid (alpha-1-acidic glycoprotein) was coupled to Pseudomonas 7A glutaminase-asparaginase by glutaraldehyde, iodinated and injected into mice. The half-life of radioactivity and glutaminase activity in plasma was about 7 min. Radioactivity and glutaminase activity in the liver reached a peak at about 20 min. The radioactivity in liver then declined with a half-life of about 20 min. Enzyme activity in liver declined with a half-life of about 10 min. The ratio of enzyme activity to radioactivity was lower in the liver than in plasma at all times during the experiment, indicating rapid hepatic inactivation of the enzyme. Uptake into the liver could be blocked by excess desialised orosomucoid. Glutamine levels in the liver were about 10% of normal for 44 min but returned to 50% of normal by 93 min. Intestines, kidney and spleen failed to exhibit any appreciable uptake of desialated orosomucoid glutaminase-asparaginase.

The incorporation of homologous and heterologous hypoxanthine-guanine phosphoriboxyltransferase into mutant cells

Experiments are described leading to partial compensation of a deficiency in the enzyme hypoxanthine-guanine phosphoribosyltransferase in mutant cells by supplying the cells with exogenous purified enzymes. DEAE-dextran is an effective helper agent, whereas poly (L-lysine), lysolecithin and amphotericin B seem to inhibit the entry of the enzymes of their activity. Enzyme preparation from Chinese hamster was found to have different effects in different mutant cell lines. In mutant Chinese hamster cells, the electrophoretic activity pattern remains unchanged for the Chinese hamster enzyme, but changes progressively to faster-moving activity peaks for the human enzyme after several hours. The metabolic effect of the incorporated enzyme is in the range between 3 and 4% of the normal cellular enzyme activity which corresponds to a 10--20 fold increase of hypoxanthine-guanine phosphoribosyltransferase activity in the mutant cells.

alpha-galactosidase A from human placenta. Stability and subunit size

alpha-Galactosidase A (alpha-D-galactoside galactohydrolase, EC 3.2.1.22) was purified from human placenta. The purified enzyme showed one major band on polyacrylamide gel electrophoresis and a single precipitin line on double immunodiffusion. Electrophoresis of the purified, S-carboxymethylated enzyme on sodium dodecyl sulfate polyacrylamide gel showed one component with a molecular weight of about 65 000, but electrophoresis of the non-S-carboxymethylated enzyme showed two components, a major band with a molecular weight of 67 500 and a diffuse band with a molecular weight of 47 000. We suggest that the smaller diffuse component is a degradation product and that the enzyme is a dimer with a molecular weight of approximately 150 000 and a subunit of molecular weight of about 67 500. Antibody raised against the purified enzyme quantitatively precipitated alpha-galactosidase A, but not alpha-galactosidase in Fabry's disease fibroblasts. The alpha-galactosidase A is very heat labile and pH sensitive. It is most stable in concentrated solution at low temperature and at a pH of 5.0 to 6.0. When added to plasma at 37 degrees C, it has a half-life of only 17 min. This imposes a serious obstacle to its use in the treatment of Fabry's disease.

Introduction of artificial crosslinks into proteins

In this chapter, we present a brief overview of the current status of protein crosslinking technology. An attempt is made to compare the natural crosslinks and natural crosslinking agents to the artificial ones, and a brief section is devoted to the potential use of enzymes (transglutaminase and peroxidase) as crosslinking agents in vitro. Homobifunctional (x-R-x) and heterobifunctional (x-R-y) reagents are considered in terms of the kinds of functional groups and R-groups that have been used in protein crosslinking, and some examples of reagents and applications from the recent literature are tabulated.